Child maltreatment is a critical public health concern. Individuals maltreated in childhood are at risk for a number of serious psychiatric and medical problems over the lifespan. The idea that maltreatment heightens risk for disease via immune dysfunction has been fueled by the results of numerous studies showing that, among adults exposed to maltreatment in childhood, biomarkers of systemic inflammation are elevated. Yet the extent to which systemic inflammation is present during childhood or adolescence is unclear. Findings are mixed, with some studies showing support for the relation between maltreatment and inflammation in youth, and others showing null results. There is some empirical evidence, however, that when social support is considered, the variance in systemic inflammation among youth is better explained. Among adolescents recently exposed to a negative life event, those with little social support had significantly more systemic inflammation compared to the adolescents with adequate social support. These findings provide evidence of social support as a protective factor for youth exposed to normative life events. How social support operates for youth with non-normative, chronic, and potentially life threatening trauma is unknown. This is a significant omission in the field given that youth with severe and/or chronic maltreatment are at greatest risk for health problems. Further, these youth may be removed from their homes, placed in multiple foster placements, and reunited with biological caregivers several times over the course of their childhoods, resulting in fluctuating social support conditions. Capturing the pattern of shifting social support is essential to understanding how psychosocial factors relate to systemic inflammation in foster youth, a transient population whose lifetime exposure to maltreatment greatly exceeds what is typical, putting them at risk for poor health now and later in life. The proposed study will utilize a longitudinal structural equation modeling framework to test the interaction of maltreatment and social support over time on inflammatory biomarkers (i.e., interleukin-6 and C-reactive protein) in foster youth (aim 2). Participants will be drawn from a larger, longitudinal R01, entitled ?Studying Pathways to Adjustment and Resilience in Kids? (SPARK) and will include 220 youth ages 10 to 18 years. Participant recruitment and data collection will be conducted in coordination with the sponsor's new R01, which will also utilize previous SPARK families. The current study will improve on past methodological weaknesses by including empirically-derived measurement models and examining correlates of the constructs of interests for inclusion as covariates (aim 1). Findings will address a major gap in the literature and contribute a crucial missing piece to the question of how maltreatment confers risk for expensive and devastating mental and physical health problems over the life course.